In general, a spark plug that is used for ignition of an internal combustion engine such as automobile engines has a structure where an insulator made of an alumina ceramic, etc. is disposed inside a metal shell installed with a ground electrode, and a center electrode is disposed inside the insulator. The insulator projects from the rear opening of the metal shell to the axial direction. A terminal metal fixture is disposed inside the projecting part of the insulator and is connected to the center electrode via a conductive glass seal that is formed in the glass sealing step. And a high voltage is applied to the terminal metal fixture to cause spark discharge over a gap formed between the ground electrode and the center electrode.
However, under some combined conditions where, for example, the temperature of the spark plug increases, and the circumferential temperature increases, it may happen that even when a high voltage is applied to the spark plug, normal discharge does not occur over the gap, but discharge occurs between the terminal metal fixture and the metal shell on the surface of the insulator to cause dielectric breakdown (this phenomenon will be hereinafter referred to as “flashover”). For that reason, in most of commonly used spark plugs, for the sake of mainly avoiding this flashover phenomenon, a glaze layer is formed on at least a part of the surface of the insulator. Also, the glaze layer serves to smoothen the insulator surface, thereby preventing contamination, and to enhance chemical or mechanical strength of the insulator.
In the case of alumina based insulators for spark plug, there have hitherto been used glazes of lead silicate glass where a silicate glass is compounded with a relatively large amount of PbO to lower the softening point. In recent years, however, glazes containing a Pb component have been losing acceptance from the standpoint of environmental protection. Then, for the sake of making a glaze leadless, there have been made various reviews.
As a background art, there is JP-A-2001-39733.